Bug 66425: Avoid a NullPointerException found via oss-fuzz

We try to avoid throwing NullPointerException, but it was possible
to trigger one here with a specially crafted input-file

Should fix https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=62151

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1912198 13f79535-47bb-0310-9956-ffa450edef68

poi-ooxml/src/main/java/org/apache/poi/xslf/usermodel/XSLFTable.java

@@ -114,7 +114,7 @@ public class XSLFTable extends XSLFGraphicFrame implements Iterable<XSLFTableRow
 
     @Override
     public int getNumberOfColumns() {
-        return _table.getTblGrid().sizeOfGridColArray();
+        return _table.getTblGrid() == null ? 0 : _table.getTblGrid().sizeOfGridColArray();
     }
 
     @Override